The present invention relates to a real-image finder optical system and an apparatus using the same. More particularly, the present invention relates to a real-image finder optical system having an image-inverting optical system that is suitable for use in still cameras, still video systems, etc. in which a photographic optical system and a finder optical system are provided separately from each other. The present invention also relates to an apparatus using the real-image finder optical system.
In lens-shutter cameras and so forth, a photographic optical system and a finder optical system are provided separately from each other. Finder optical systems used in such apparatus may be roughly divided into virtual-image finders and real-image finders.
Virtual-image finders have the disadvantage due to the arrangement thereof that the diameter of the front lens is unfavorably large and the visibility of the view frame is not good. Accordingly, virtual-image finders involve a serious problem in achieving compact and high-performance finder optical systems. In contrast, real-image finders have an arrangement in which a view frame is placed in the vicinity of an intermediate image plane of an objective optical system, and the view frame is observed through an ocular optical system. Therefore, the boundaries of the view frame can be seen clearly. Moreover, because the position of the entrance pupil is close to the object side thereof, the objective optical system can be reduced in size in the diametric direction. Therefore, many of lens-shutter cameras that are stated to be "compact and high-performance" employ real-image finders.
To achieve a further reduction in the size of real-image finders, many of recent real-image finders employ a technique whereby the focal length of the objective optical system is reduced with the necessary field angle ensured, i.e. the height of the intermediate image is reduced. Consequently, the ratio between the focal lengths of the objective optical system and the ocular optical system, that is, the finder magnification, is sacrificed in the present state of the art. Accordingly, although they are stated to be "compact and high-performance real-image finders", the image actually observed is very small and hence difficult to see.
The image to be observed can be enlarged if it is possible to simply reduce the focal length of the ocular optical system and to correct various aberrations produced therein. However, even if aberration correction can be made satisfactorily, there is a limit to the reduction in the focal length of the ocular optical system because of the need to ensure the optical path length required for the image-inverting optical system. Moreover, many of ocular optical systems generally used are single-lens optical systems. Therefore, if the focal length of the ocular optical system is reduced, aberrations, particularly chromatic aberrations, are aggravated and become impossible to correct.
Under these circumstances, there have recently been made some propositions that a curved surface is used to form a reflecting surface of an image-inverting optical system of a real-image finder, that is, a reflecting surface of a prism or a mirror that constitutes the image-inverting optical system, thereby giving a power to the reflecting surface of the image-inverting optical system. However, a reflecting surface of the image-inverting optical system is generally decentered with respect to the optical axis. Therefore, if a power is given to the decentered reflecting surface, aberrations due to decentration that are rotationally asymmetric are produced. The decentration aberrations are basically impossible to correct by a rotationally symmetric surface alone.
Japanese Patent Application Unexamined Publication Number [hereinafter referred to as "JP(A)"] 8-248481 uses a rotationally symmetric curved surface as a reflecting surface of a prism that forms a real-image zoom finder of a lens-shutter camera. It is stated in the publication that an aspherical surface or a toric surface is applicable to the curved surface. However, the aspherical surface disclosed in the specification of the publication is rotationally symmetric. The toric surface is also symmetric with respect to two coordinate axes. Therefore, correction for skew rays cannot satisfactorily be performed. In either example, a curved surface is used as a reflecting surface of a prism. However, the prism is placed closer to the object side than the intermediate image, and there is no intention of reducing the focal length of the ocular optical system.
JP(A) 9-152646 uses a rotationally asymmetric curved surface as a reflecting surface of a prism that forms a real-image finder of a lens-shutter camera for a fixed-focal length lens. As stated in the specification, the prism is placed closer to the object side than the intermediate image in order to function as an objective lens. There is no intention of reducing the focal length of the ocular optical system.
EP0722106A2 discloses the same subject matter as that of the above-described JP(A) 8-248481 and 9-152646.
JP(A) 8-292368, 8-292371, 8-292372, 9-5650, 9-90229, 9-211330, 9-211331, 9-222561, 9-258105 and 9-258106 disclose examples in which image inversion is performed in single focal length and zoom image pickup apparatuses by using a prism optical system having a rotationally asymmetric surface. In some examples, the disclosed arrangement is expressed as applicable to a finder optical system. However, none of them are concerned with a real-image finder. These conventional techniques have no intention of reducing the focal length of an ocular optical system while ensuring the optical path length of the image-inverting optical system by giving a power to a reflecting surface of a prism.